Figure 4: Regeneration limit.

(a) Gain, the regenerative Shannon capacity ratio to the Shannon formula C_L=log₂(1+ρ), for the different number of regenerators. The analytical results shown by black (dashed) equation 5 and dotted using equation 6) lines demonstrate an excellent agreement with numerics (solid coloured lines). The inset shows mutual information gain for M²—rectangular constellations approaching capacity gain. (b) Capacity and mutual information for discrete M²—rectangular (left panel) and M—points in the ring (right panel) alphabets. The arrows show the constant capacity increase. Constellations and associated attraction regions are shown.